Recently, in the background of mounting concern about environmental problems and social demand for advanced medical care, there is an increasing request for technology and apparatus capable of analyzing traces of chemical substance or biological substance easily. Owing to the advantages in cost, ease of handling and shortening of measuring time, as compared with the conventional analytical technology, intensive studies are actively conducted in the field of micro total analysis system (also referred to as μTAS or Lab-On-Chip).
In the micro total analysis system, a series of chemical operations such as sample mixing, reaction and separation are “micronized” and integrated on a glass or plastic substrate. Previously, studies of the micro total analysis system are mainly about handling of sample liquid as continuous fluid, but recently, the studies for handling liquid as droplets have attracted attention because pump and valve are not required and power consumption is smaller (US 2004/0058450 (Patent Document 1), Japanese Patent Application Laid-Open Publication No. 10-267801 (Patent Document 2), Applied Physics Letters, Vol. 77, No. 11, pp. 1725-1726 (Non-Patent Document 1), Journal of Applied Physics, Vol. 92, No. 7, pp. 4080-4087 (Non-Patent Document 2), Proc. MEMS2003, pp. 694-697 (Non-Patent Document 3), and IEEE Industry Applications Society, Annual meeting, New Orleans, La., Oct. 5-9, 1997, “Electrical actuation of liquid droplet for microreactor applications” (Non-Patent Document 4)).
One of the methods for handling liquid as droplet is known as Electrowetting. Electrowetting is a technology for controlling the wetting of liquid on the solid surface by the application of voltage, and the principle of conveying droplets is described as Electrocapillarity or Electromoistening in Non-Patent Documents 1 and 2 and Patent Document 1.
M. G. Pollack et al. have invented a device in Non-Patent Document 1, in which a lower substrate having a plurality of electrodes for control on its flat surface and an upper substrate having a ground electrode on its flat surface are disposed in parallel with interposing a gap therebetween, the gap is filled with silicone oil, and droplets of electrolyte are put therein. The Non-Patent Document 1 has reported that, by changing over the switches coupled to the plurality of electrodes for control, potentials of the electrodes for control are controlled, and the droplets of electrolyte existing in the gap between the substrates filled with silicone oil are conveyed with the applied voltage of 40 V to 80 V. At this time, the plurality of electrodes for control on the lower substrate are covered with a dielectric layer (parylene, thickness: 700 nm), and the surface thereof is covered with water repellent substance (Teflon (registered trademark), thickness: 200 nm). Further, the ground electrode on the upper substrate is also covered with water repellent substance (Teflon (registered trademark), thickness: 200 nm). Also, M. G. Pollack et al. have described a device having ground electrode and control electrode on the same substrate and a conveying mechanism on one side in Patent Document 1.
As an example where liquid droplets are conveyed in the air without using the silicone oil as the filler in the same structure as in Non-Patent Document 1, a device by H. Moon et al. is known. In Non-Patent Document 2, H. Moon et al. have reported that droplets can be conveyed with the applied voltage of 15 V, by making use of a high dielectric material such as BST (Barium Strontium Titanate) as the dielectric.
The devices by M. G. Pollack et al. and H. Moon et al. are devices for moving the droplets in a one-dimensional direction. However, in Non-Patent Document 3, S.-K. Fan et al. have reported the development of EWOD (Electro Wetting On Dielectric) liquid delivery device, in which a lower substrate having N rectangular electrodes and an upper substrate having M rectangular electrodes are combined so that the corresponding electrodes are arranged at a right angle, and the droplets are moved to the positions of N×M lattice points composed of the upper and lower electrodes.
As another method in which liquid is handled as droplets, the method, in which the Maxwell stress distribution on the droplet surface is changed by switching the potentials of electrodes present below the droplets, thereby conveying the droplets, is known.
In Non-Patent Document 4, by using a device having a plurality of electrodes on its flat surface and sequentially switching the potentials of the electrodes, Washizu has successfully conveyed droplets present on the device in a one-dimensional direction with the applied voltage of 400 Vrms. At this time, the plurality of electrodes on the substrate are covered with a dielectric layer (SC450 (registered trademark), thickness: 10 μm), and the surface thereof is covered with water repellent substance (Teflon (registered trademark)). Further, in Patent Document 2, Washizu has described a structure in which a pipe with a water repellent surface is provided on a device having a plurality of electrodes on its flat surface.